1. Field of the Invention
The present invention relates to a unidirectionally operating laser apparatus using a semi-monolithic ring cavity having a structure including a laser active medium and an optically active polarization rotator or half-wave plate, along with a conventional mirror adapted to generate a continuous-wave, wavelength-tunable laser output, thereby being capable of operating at a single longitudinal mode while achieving a frequency tuning and modulation at a high speed and a high power stability.
2. Description of the Prior Art
Generally, laser devices have two mirrors to constitute a laser cavity. In such a conventional laser device, a spatial hole burning phenomenon, caused by a standing-wave distribution of the laser intensity in the cavity, occurs. Such a spatial hole burning phenomenon prevents a laser device from operating at a single frequency in its homogeneously line-broadened laser medium. As a result, several longitudinal modes oscillate simultaneously in the laser cavity. In order to eliminate the formation of a standing wave in the laser medium, it is required to make a laser beam travel in a single direction in the laser cavity. Therefore, a laser device has been proposed which includes a ring cavity using three or four mirrors. Usually an optical diode is inserted in the ring cavity to achieve a unidirectional operation.
Such an optical diode serves to generate a higher optical loss in one of laser waves traveling in two different directions in the ring cavity. The laser wave with a higher optical loss results in no laser oscillation. Accordingly, laser oscillation occurs in the direction along which the laser wave with a lower optical loss travels. The optical diode consists of a reciprocal polarization rotator, a nonreciprocal polarization rotator, and a polarizer. The reciprocal polarization rotator may include an optically active polarization rotator or a half-wave plate. The nonreciprocal polarization rotator may include a Faraday rotator. On the other hand, the polarizer may include an optical element capable of generating a higher optical loss in one of two polarization states orthogonal to each other. To this end, conventional polarizers may be used. Alternatively, a laser rod with a Brewster angle may be used.
When a laser beam with a certain polarization state passes through a polarization rotator, it is transformed into a polarization state different from its original polarization state. When the transformed laser beam passes reversely through the polarization rotator, its original polarized state may be recovered in accordance with the feature of the polarization rotator used. Such a polarization rotator is called a "reciprocal polarization rotator.
FIG. 4 illustrates operation characteristics of a half-wave plate which is a reciprocal polarization rotator. In FIG. 4, the half-wave plate is denoted by the reference numeral 21. Where a laser beam with a linear polarization state P.sub.1 is transformed into a linear polarization state P.sub.2 as it rotates by an angle .gamma. while traveling in a direction from left to right through the half-wave plate 21, its polarization state is recovered from the state P.sub.2 to its original polarization state P.sub.1 when it travels in a direction from right to left through the half-wave plate 21 because it rotates reversely by the angle .gamma..
In the case of a nonreciprocal polarization rotator, no recovery of the polarization state of a laser beam transformed after passing through the polarization rotator in one direction occurs after the laser beam passes reversely through the polarization rotator. Rather, the same transformation as the first transformation occurs again.
FIG. 5 illustrates operation characteristics of a Faraday rotator which is a nonreciprocal polarization rotator. In FIG. 5, the Faraday rotator is denoted by the reference numeral 23. Where a laser beam with a linear polarization state P.sub.1 is transformed into a linear polarization state P.sub.2 as it rotates by an angle .gamma. while traveling in a direction from left to right through the Faraday rotator 23, its polarization state is transformed again from the state P.sub.2 to a polarization state P.sub.3 when it travels in a direction from right to left through the Faraday rotator 23 because it rotates again by the angle .gamma..
Referring to FIG. 6, a well-known unidirectionally-operating laser cavity with four mirrors is illustrated [Reference: Walter Koechner, Solid-State Laser Engineering (Springer, N.Y., 1996) Chap. 3, p.140 and the reference therein]. As shown in FIG. 6, the laser cavity uses an optical diode which consists of a half-wave plate 21, a Faraday rotator 23, and a laser medium with a Brewster angle 24. Assuming that the half-wave plate 21 and Faraday rotator 23 are configured to rotate the plane of polarization of a wave traveling clockwise by angles of .beta. and .theta., respectively, they rotate the plane of polarization of a wave traveling counterclockwise by angles of .beta. and -.theta., respectively. In this case, accordingly, the difference, .DELTA..alpha., in the power loss generated in a polarizer between two waves traveling in opposite directions corresponds to "sin.sup.2 (.beta.+.theta.)-sin.sup.2 (.beta.-.theta.)" [.DELTA..alpha.=sin.sup.2 (.beta.+.theta.)-sin.sup.2 (.beta.-.theta.)]. Therefore, the laser device oscillates in a direction with a lower power loss, namely, counterclockwise. In FIG. 6, the reference numeral 25 denotes a laser rod which is used for the polarizer.
Generally, continuous-wave tunable laser devices are classified into those of an open cavity and those of a monolithic cavity.
The open cavity uses four or three mirrors to constitute a planar ring cavity. Optical components such as a laser active medium, a Faraday rotator, a half-wave plate and a polarizer are inserted in the planar ring cavity. The laser medium may be configured to have Brewster angles at opposite ends thereof, so that it serves as a polarizer.
In the case of the open cavity, the four or three mirrors constituting a ring cavity are separated from the laser medium. The Faraday rotator and half-wave plate, which serve as an optical diode, are inserted in the cavity such that they are positioned on an optical path defined in the cavity. Since the open cavity consists of several optical components, it is very bulky as compared to the monolithic cavity. Furthermore, the open cavity is mechanically instable because the optical components thereof are mounted to different optical mounts, respectively. This results in considerably low laser power and frequency stability.
On the other hand, the monolithic cavity is a cavity in which a single laser active medium has all functions of the Faraday rotator, half-wave plate and polarizer. This monolithic cavity has a non-planar ring cavity structure.
In other words, the monolithic cavity has a structure capable of eliminating drawbacks involved in the open cavity. Since all optical components of a ring cavity required for a unidirectional operation are formed integrally into a single laser active medium, the entire structure is very compact and mechanically stable. Accordingly, considerably high laser power and frequency stability are obtained. In such a monolithic cavity, however, there is no optical component enabling a frequency tuning in any portion of the monolithic cavity except for the laser active medium. To this end, frequency tuning is achieved in this monolithic cavity by mainly controlling the temperature of the laser active medium. However, such a frequency tuning and modulating method based on the temperature control is carried out at a very low speed because it depends on a time constant of the active medium. In addition, it is required to fabricate the laser active medium in order to make a non-planer ring cavity. This results in a difficulty in the design and fabrication of the non-planar ring cavity as compared to planar ring cavities.